This Program Project activity involves several interrelated studies of various aspects of heart transplantation. No clinical heart transplants are involved at the present time in the Program. Efforts will be directed toward improving the control of transplant rejection by the individualization of immunosuppressive treatment, especially with anti-thymocyte globulin, through the use of serial tests for T cells in patients receiving kidney transplants and animals receiving heart transplants. These will involve characterization of infiltrating leukocytes in transplants in collaboration with the immunopathologists. Studies on the assessment and the control of states of preexisting immunity to transplants will be pursued and the immunogenetics of interspecies transplants will be evaluated in certain animals. The development of techniques to transplant hearts in sheep will continue and physiological studies on isolated heart preparations, including the maintenance of excised, perfused hearts, will be extended.